HR: 1330h
AN: V12B-0584    [PDF]
TI: Age Progression and Changes in Mantle Source Composition With Time in the Rano Rahi Seamount Field, East Pacific Rise Flank, $15\deg$-${19\deg$S
AU: * Hall, L S
EM: lindahall@seanet.com
AF: SOEST, U of Hawaii, Honolulu, HI 96822 United States
AU: Mahoney, J J
EM: jmahoney@hawaii.edu
AF: SOEST, U of Hawaii, Honolulu, HI 96822 United States
AU: Sinton, J M
EM: sinton@hawaii.edu
AF: SOEST, U of Hawaii, Honolulu, HI 96822 United States
AU: Duncan, R A
EM: rduncan@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 United States
AB: The Rano Rahi seamount field, located on the flanks of the East Pacific rise at $15\deg$-$19\deg$S, has unusually long chains of numerous seamounts in its southern half ($17\deg$-$19\deg$S), extending south and east from the eastern end of the Pukapuka ridge ($17.2\deg$S, $118.5\deg$W) to the ridge. To examine the variation in space and time of the seamounts' mantle source composition, Sr, Nd, and Pb isotopic ratios, $^{40}$Ar-$^{39}$Ar ages, and incompatible element concentrations of representative lavas were measured. Isotopic and incompatible element ratios overlap the linear trends of ratios from both the nearby ridge and the Pukapuka ridge (Mahoney et al., 1994; Janney et al., 2000), implying that seamount lavas' mantle sources were composed of varying proportions of the same two components (N-MORB and C-type mantle) making up the sources of Pukapuka ridge and axial lavas. $^{40}$Ar-$^{39}$Ar incremental-heating plateau ages of seamount lavas range from 0.23 - 4.38 Ma. Lavas recovered farther from the ridge are generally older than near-ridge lavas, implying that the seamount field represents a time sequence, rather than a current map, of mantle source compositions. Fewer samples were dated than were analyzed for composition, but the increasing ages of dated samples with distance from the ridge allowed the ages of many of the undated lavas to be estimated using the seamounts' current distance from the ridge, the spreading rate, and the dated lavas' average distance at eruption. This in turn allowed the use of most of the undated lavas to asssess changes in composition with time. In the southernmost seamount chains, about $18\deg$-$19\deg$S, isotopic and incompatible element ratios did change with time: ratios in seamount lavas older than 4 Ma are similar to those of the youngest Pukapuka ridge lavas; from about 4 Ma seamount Sr and Pb isotopic ratios increased ($\epsilon_{Nd}$ decreased), culminating at 1-2 Ma, while ratios such as Nb/Zr decreased slightly (i.e., isotopic and incompatible element ratios were decoupled in these lavas). These samples have some of the most C-like isotopic ratios in the seamount field. In contrast, lavas from seamount chains at $17\deg$-$18\deg$S have no compositional trend with time. Instead, compositions of these lavas tend to be less C-like toward the north, in spite of evidence that the mantle source under the ridge at about $17.3\deg$S is currently the most C-like (Mahoney et al., 1994). In addition, although the long seamount chains occur south of $17\deg$S, geophysical indications of the presence of melt in the mantle are greatest at about $16\deg$-$17\deg$S (Forsyth et al., 1998). We suggest that this mismatch between the seamount chains and the current region of high melt production may be reconciled by a scenario in which the seamount chains are the result of two phenomena, the intersection of the Pukapuka ridge with the East Pacific Rise at about 7 Ma, and subsequent extension of the Pacific plate at its thin edge near the ridge. We examine two possible causes of extension: asthenospheric pressure exerted from the west by the South Pacific Superswell (Toomey et al., 2002), and stress caused by a new northward component of Pacific plate motion since 6 Ma (Wessel and Kroenke, 2000).
DE: 1025 Composition of the mantle
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
DE: 5480 Volcanism (8450)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting